1. Field of the Invention
The present invention relates to a solid electrolyte battery incorporating a solid electrolyte, and more particularly to a solid electrolyte battery incorporating a separator which has specific mechanical strength and thermal characteristics so as to considerably improve its energy density and safety.
2. Description of the Related Art
As a power source for a portable electronic apparatus, such as a portable telephone or a notebook personal computer, a battery is an important element. To reduce the size and weight of the electronic apparatus, increase in the capacity of the battery and reduction in the volume of the same have been required. From the foregoing viewpoints, a lithium battery exhibiting a high energy density and output density is suitable to serve as the power source of the portable electronic apparatus. A lithium battery incorporating a negative electrode made of a carbon material has a mean discharge voltage of 3.7 V or greater. Moreover, deterioration caused from charge and discharge cycles can relatively satisfactorily be prevented. Therefore, the lithium battery has an advantage that a high energy density can easily be realized.
Lithium batteries are required to permit a variety of shapes to be formed. The batteries have the flexibility and high degree of freedom of their shape to form a sheet battery having a small thickness and a large area and a card battery having a small thickness and a small area. A conventional structure, in which battery elements—a positive electrode, a negative electrode, and an electrolytic solution—are enclosed in a metal can, encounters difficulties in forming the variety of shapes. Since the electrolytic solution is employed, the manufacturing process becomes too complicated. Moreover, a countermeasure against leakage of the solution must be taken.
To solve the above-mentioned problems, batteries have been researched and developed which incorporate a solid electrolyte composed of either a conductive organic polymer or inorganic ceramic solid electrolyte or a gel-like solid electrolyte (hereinafter called a “gel electrolyte”) in which matrix polymers are impregnated with electrolytic solution. In both types of solid electrolyte batteries the electrolyte is fixed. Therefore, contact between the electrode and the electrolyte can be maintained. Hence it follows that the foregoing batteries are free from the necessity of having to enclose the electrolytic solution by employing a metal can or by exerting pressure on the battery element. A film-shape case material can be used to reduce the thickness of the battery. Thus, an energy density greater than that of a conventional battery can be realized.
In general, the solid electrolyte of the solid electrolyte battery has proper mechanical strength as disclosed in “MATERIAL TECHNIQUE OF HIGH-PERFORMANCE SECONDARY BATTERY AND EVALUATION, APPLICATION AND DEVELOPMENT OF THE SAME” (Technical Information Association, 1998). Therefore, a structure of the battery, distinct from that of the conventional battery incorporating the electrolytic solution, can be selected. For example, it has been reported that a separator is not required between the positive electrode and the negative electrode. This provides a known advantage for the solid electrolyte.
The reported solid electrolyte suffers from unsatisfactory strength including piercing resistances as compared with the conventional separator constituted by a polyolefin porous film and the like. When the thickness of the solid electrolyte of the conventional solid electrolyte battery is reduced to, for example, 40 μm or greater to raise the energy density, there arises a problem in that internal short circuiting frequently occurs after the battery has been assembled. As described above, the energy density of the solid electrolyte battery cannot easily be raised by reducing the thickness of the solid electrolyte layer.
As for heat resistances which is an index to evaluate the reliability of the battery, the conventional solid electrolyte battery suffers from unsatisfactory heat resistance. A portion of the batteries on the market are designed to use a so-called “shutdown effect” to improve heat resistance. However, a solid electrolyte material for the solid electrolyte battery having the shutdown effect has not been found.
As for the reliability and safety of the battery, the reliability and safety cannot easily be realized as the energy density of the battery is raised. Therefore, a technique for maintaining the safety of the solid electrolyte battery also must be considered when a solid electrolyte battery is designed to raise the energy density.
A thin battery incorporates a separator which is made of polyolefin. In particular, a polyethylene separator is employed.
In a usual state, when the temperature of the battery has melted down and, therefore, short circuiting occurs between the positive electrode and the negative electrode, thermorunaway does not occur. In a case where a battery is used in an abnormal environment, for example, in a case where the temperature of a battery has been raised because the battery has been charged to a voltage level greater than a usual level, there is apprehension that an accident can occur. In the foregoing case, there is apprehension that use of a separator made of polyethylene which has a melting point less than that of polypropylene, might cause a melt-down of the separator to take place. That is, breakage of the separator might occur, causing a short circuit between the positive electrode and the negative electrode to take place. Thus, there is apprehension that the battery will generate heat.